Plastic packaging has certain inherent benefits over glass and metal packaging, such as light weightability, increased variability in package design, non-breakability, and reduced cost. However, plastic packaging may have greater permeability to certain gases (oxygen and carbon dioxide) and liquids (water) than glass or metal; these gases/liquids permeate the plastic and reduce the shelf life of the product contained therein. Various specialty polymers and layer structures have been developed which provide a commercially-acceptable shelf life for some oxygen-sensitive products, such as juice and ketchup.
There are two general types of oxygen-barrier materials—passive and active. A “passive” barrier retards oxygen permeation into the package. For example, with multi-layer technology it is possible to incorporate thin layers of expensive barrier polymers (e.g., polyvinylidene chloride copolymer (PVDC) or ethylene vinyl alcohol copolymer (EVOH)), in combination with structural layers of bottle-grade plastic resins (e.g., polyethylene terephthalate (PET)), to provide a cost-effective barrier package.
In an “active” barrier package, an oxygen “scavenger” is incorporated into a single or multi-layer plastic structure to theoretically remove the oxygen initially present and/or generated from the inside of the package, as well as to retard the passage of exterior oxygen into the package. Thus, oxygen-scavengers are superior to passive barriers in that they both remove oxygen from inside the package and retard its ingress into the package. However, the performance of the prior active barrier packages is reported in terms of an overall ingress such that the oxygen content continues to increase over time, albeit at a slower rate of increase than with some passive barriers.
Commercially successful hot-fill juice containers (passive barrier) have been developed by Continental PET Technologies, Inc. of Florence, Ky., which provide a 1.5 to 4-time improvement in oxygen barrier property over a standard commercial single-layer PET container. These multi-layer juice containers include two very thin intermediate barrier layers of EVOH positioned between inner and outer layers of virgin PET, and a core layer of either virgin or recycled PET. However, there are products even more “oxygen sensitive” than juice—e.g., beer. The taste of beer deteriorates rapidly in the presence of oxygen and thus beer requires at least a 10-times greater oxygen barrier property than provided by the standard single-layer PET container. Furthermore, beer's oxygen sensitivity is enhanced by increased temperature, i.e., exposure to heat during storage has a multiplicative impact on oxygen's adverse effect on taste. For example, if beer is refrigerated during storage and the amount of oxygen is maintained below a specified parts-per-billion (ppb), a given container may have a shelf life of 4-6 weeks (28-42 days). However, if the same beer container is not refrigerated, then the shelf life may be reduced to 1-2 weeks (7-14 days).
One possible solution for highly oxygen-sensitive products is to utilize higher barrier polymers in packaging. For example, polyethylene naphthalate (PEN) has a 5-time improvement in oxygen barrier property over polyethylene terephthalate (PET). Also, PEN has a significantly higher glass transition temperature (Tg) than (PET)—about 120° C. compared to 80° C.—and thus PEN is also desirable for use in thermal-resistant (e.g., pasteurizable) beer containers. However, PEN is more expensive than PET (both as a material and in processing costs), and thus the improvement in properties must be balanced against the increased expense. Also, the increase in passive barrier protection with PEN does not solve the problem of residual oxygen within the package.
One method of achieving a package that is lower in cost than PEN, but with higher barrier and thermal properties, is to provide a blend of PEN and PET. However, blending of these two polymers generally results in an opaque material (incompatible phases). Efforts to produce a clear (transparent) container or film from a PEN/PET blend, and maintain strain hardening (for structural strength) have been ongoing for over ten years but there is still no commercial process in wide-spread use for producing such articles.
Another possible solution, on which extensive work has been reported, is the use of alleged metal-activated oxidizable organic polymers (e.g., polyamides) as oxygen-scavengers in plastic containers. However, problems again exist with: lack of clarity; time/expense required to activate the scavenging polymer; toxicity of the metal; need to prevent interaction of oxidative reaction byproducts with the package contents and/or environment; and loss of the oxygen-scavenging effect during storage (prior to filling). For example, U.S. Pat. No. 5,034,252 to Nilsson suggests a single-layer container wall consisting of a blend of PET, 1-7% by weight polyamide (e.g., MXD-6 nylon), and 50-1000 ppm (parts-per-million) of a transition metal (e.g., cobalt). Nilsson theorizes that cobalt forms an active metal complex having the capacity to bond with oxygen and to coordinate to the groups or atoms of the polymer. However, Nilsson notes that low-oxygen permeability coefficients are achieved only after an aging (activation) process, which may require exposure of the preform/container to a combination of temperature and humidity. U.S. Pat. No. 5,021,515 to Cochran generally describes the use of a PET/polyamide blend with cobalt. It suggests a multi-layer structure formed by coextrusion lamination using adhesive tie layers, wherein inner and outer layers prevent interaction of a central scavenging layer (containing cobalt) with the package contents and environment. However, Cochran similarly notes the aging effect.
A significant problem with blending polyesters (such as PET) and polyamides (such as MXD-6 nylon) is loss of clarity. Most food manufacturers require the transparency of a PET container, and will not accept a loss of transparency in order to achieve a desired oxygen-barrier property.
Thus, the prior art containers typically suffer from one or more of the following difficulties:                (a) lack of transparency;        (b) inability to process the polymers on commercial injection molding equipment;        (c) only marginal improvement in oxygen-scavenging performance over monolayer PET bottles or multilayer PET/EVOH bottles;        (d) aging or activation requirement to induce oxygen-scavenging performance;        (e) high cost and/or toxicity.        
One reference discloses that under test conditions designed to approximate the actual conditions in beverage applications, the scavenging performance of a plastic container having a scavenging polyamide was “comparable” to a glass bottle (see for example, the PET/polyamide blend of U.S. Pat. No. 5,021,515, example 8). In reality, plastic containers having a performance only comparable to glass do not provide a significant incentive for beer producers to give up their substantial investment in glass container bottling operations. Still further, the same blend art states that with increasing concentrations of the metal, the oxygen-scavenging performance actually decreases. Again, this would discourage one from believing that oxidizable polymers (such as polyamide) could provide a commercial container which satisfied the stringent low-oxygen requirements for beer.
The variety of oxygen-barrier systems disclosed in the art is strong evidence of the commercial need for such packaging, and also that the known systems have not solved many of the problems. Thus, there is an ongoing need for oxygen-scavenging polymers having enhanced scavenging capacity and for a process to manufacture transparent articles from such polymers in a cost-effective manner.